1. Technical Field of the Invention
The present invention relates to an apparatus for calculating a residual capacity of a secondary battery, such as a lead battery, nickel-cadmium battery, nickel-hydrogen battery, lithium secondary battery, lithium-ion secondary battery, or a lithium-ion secondary battery having a positive pole that contains lithium metal phosphate having an olivine structure.
2. Related Art
It is known that, in detecting a residual capacity, or a state of charge (SOC), of energy, in a secondary battery, an apparatus for detecting a residual capacity is used as disclosed in patent documents JP-A-2009-129644 or JP-A-2006-038494. For example, the lithium-ion secondary battery disclosed in the patent document JP-A-2009-129644 has a characteristic in which a flat charge-discharge capacity range is ensured. The flat charge-discharge capacity range accounts for 50% or more of the electrical capacity of the battery, in which a theoretical electrical capacity is the upper limit. In the flat charge-discharge capacity range, when the battery is charged/discharged with a current corresponding to 1 C, the variation of voltage applied across the terminals falls within a capacity range of 0.2 V or less in both of charging and discharging.
Thus, in the lithium-ion secondary battery or in a battery pack that includes the lithium-ion secondary battery disclosed in JP-A-2009-129644, variation in the internal resistance of the lithium-ion secondary battery is reduced for the charge/discharge of current. As a result, stable output having less variation is ensured. It should be appreciated that the battery has a positive pole formed of an olivine material causing less voltage variation and a negative pole formed of an LTO (lithium titanate) material having the similar characteristic to configure a battery cell having less voltage variation.
The apparatus for calculating a residual capacity disclosed in the patent document JP-A-2006-038494 calculates a first residual capacity based on an integrated value of the charge/discharge current of a power storage device. The apparatus also calculates a second residual capacity based on an open-circuit voltage estimated from an internal impedance of the power storage device. Then, the apparatus calculates a third residual capacity that is a final residual capacity of the power storage device. Specifically, the third residual capacity is calculated by weighting the first and second residual capacities using a weight that has been determined according the usage of the power storage device and by combining the weighted states of charge.
When the power storage device is in a state of charge at a temperature of not more than a reference temperature, the weight is adjusted such that the first residual capacity will be weighted more heavily. Also, it is so configured that the weight is determined based on the current changing rate of the charge/discharge current of the power storage device.
In other words, the residual capacity calculated based on current integration and the residual capacity calculated based on voltage estimation are weighted and combined to calculate a final residual capacity. In this case, the calculation based on voltage estimation uses an open-circuit voltage (OCV) estimated from the impedance. Also, using cell temperature T and current changing rate ΔI/Δt as parameters, the weight of the residual capacity calculated based on current integration is increased when the power storage device is in a state of charge at a temperature of not more than a predetermined temperature. In this way, a residual capacity of high accuracy is calculated making use of the advantages of both of the residual capacity based on current integration and the residual capacity based on open-circuit voltage.
In the patent document JP-A-2009-129644 mentioned above, the lithium-ion secondary battery or the battery pack that uses an olivine material is subjected to charge/discharge control under which a SOC is calculated mainly based on current integration. As mentioned above, the lithium-ion secondary battery disclosed in this patent document has an output characteristic in which a flat charge-discharge capacity range is ensured. Therefore, when high load is continuously imposed on a system, such as a vehicle or an HEMS (home energy management system), that uses the lithium-ion secondary battery or the battery pack, the calculation of a SOC based on current integration as mentioned above may result in a large error. Specifically, while high load is continuously imposed on the system, the characteristic line of charge/discharge voltage relative to SOC is flat in the flat charge-discharge capacity range, and thus the calculation of a SOC based on the voltage in the range may lead to a large error, preventing accurate calculation of a SOC.
In the apparatus for calculating a residual capacity as disclosed in the patent document JP-A-2006-038494 mentioned above, an open-circuit voltage is estimated from an impedance and then a residual capacity is calculated based on OCV-SOC characteristics stored in advance. With this process, however, an impedance measurement device is required to be added to the calculation circuit, leading to the increase in the size of the apparatus. In addition, impedance will change with the changes of temperature and current changing rate.
For this reason, the accuracy of calculating a SOC will be deteriorated in a low-temperature area or a high-current area. Meanwhile, in the apparatus, the SOC calculation is mainly based on current integration, in which the weight of a residual capacity obtained by current integration is increased at a certain cell temperature and at a certain current changing rate. As a result, there is a high probability of causing a large error in detecting a SOC under the condition where high load is continuously imposed.